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The U.S. military had been observing the submarine operate off North Korea's east coast when the vessel stopped

Heightened tensions on the Korean peninsula following a fourth North Korean nuclear test

U.S. and South Korea holding largest ever military drills

Washington (CNN)The North Korean regime lost contact with one of its submarines earlier this week, three U.S. officials familiar with the latest information told CNN.

The U.S. military had been observing the submarine operate off North Korea's east coast when the vessel stopped, and U.S. spy satellites, aircraft and ships have been secretly watching for days as the North Korean navy searched for the missing sub.

The U.S. is unsure if the missing vessel is adrift under the sea or whether it has sunk, the officials said, but believes it suffered some type of failure during an exercise.

Tensions have heightened on the Korean peninsula following a fourth North Korean nuclear test and joint U.S.-South Korean military drills.

Photos:North Korea's verbal volleys

North Korea has a history of using creative language to express loathing for its enemies. Here are some of the regime's more colorful threats against the West.March 2016: North Korea warned it would make a "preemptive and offensive nuclear strike" in response to joint U.S.-South Korean military exercises. Pyongyang issued a long statement promising that "time will prove how the crime-woven history of the U.S. imperialists who have grown corpulent through aggression and war will come to an end and how the Park Geun Hye group's disgraceful remaining days will meet a miserable doom as it is keen on the confrontation with the fellow countrymen in the north."

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Photos:North Korea's verbal volleys

March 2016: Following the imposition of strict U.N. sanctions, North Korean leader Kim Jong Un said the country's "nuclear warheads need to be ready for use at any time," the North Korean state news agency KCNA reported.

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Photos:North Korea's verbal volleys

January 2016: North Korea claimed to have successfully tested a thermonuclear weapon, justifying its right to have an H-bomb on the grounds of "self defense."

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Photos:North Korea's verbal volleys

September 2015: In a statement, North Korea said its nuclear arsenal was ready for use "at any time."

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Photos:North Korea's verbal volleys

August 2015: As forces from the U.S. and South Korea took part in joint military drills. North Korea's state media referred to the exercises, which started on August 17, as "madcap" and issued a stern warning to America: "If the U.S. ignites a war in the end, far from drawing a lesson taught by its bitter defeat in the history, the DPRK will bring an irrevocable disaster and disgrace to it."

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Photos:North Korea's verbal volleys

August 2015: On August 23, as North Korean negotiators were meeting with their South Korean counterparts over current tensions, a KCTV presenter appeared on air repeating North Korea's ambitions to "destroy the warmongering South Korean puppet military."

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Photos:North Korea's verbal volleys

December 2014: The FBI said it suspected North Korea was behind a hack of Sony Entertainment, which led executives to initially cancel the theatrical release of "The Interview." The film was a comedy about an American television personality who the CIA asks to assassinate North Korean leader Kim Jong Un. North Korea threatened "merciless" action against the U.S. if the film was released, accusing the U.S. of retaliating for the hack by shutting down North Korea's Internet access. North Korea's National Defense Commission also called U.S. President Barack Obama "reckless" and a "monkey."

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Photos:North Korea's verbal volleys

July 2014: North Korea threatens to hit the White House and Pentagon with nuclear weapons. American "imperialists threaten our sovereignty and survival," North Korean officials reportedly said after the country accused the U.S. of increasing hostilities on the border with South Korea. "Our troops will fire our nuclear-armed rockets at the White House and the Pentagon -- the sources of all evil," North Korean Gen. Hwang Pyong-So said, according to The Telegraph.

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Photos:North Korea's verbal volleys

March 2013: Angered by tougher U.N. sanctions and joint military exercises by the United States and South Korea, the Supreme Command of North Korea's military vowed to put "on highest alert" the country's "rocket units" that are assigned to strike "U.S. imperialist aggressor troops in the U.S. mainland and on Hawaii and Guam and other operational zone in the Pacific." Whether Pyongyang has the will to back up such doomsday talk is a perplexing question, but there is evidence that its know-how -- in terms of uranium enrichment, nuclear testing and missile technology -- is progressing.

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Photos:North Korea's verbal volleys

February 2013: In a message to the United States and South Korea, North Korea vowed "miserable destruction" if "your side ignites a war of aggression by staging reckless joint military exercises."

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Photos:North Korea's verbal volleys

June 2012: Once again, North Korea vowed to be "merciless" in its promised attack on the United States, this time threatening a "sacred war" as it aimed artillery at South Korean media groups. North Korea was mad that South Korean journalists had criticized Pyongyang children's festivals meant to foster allegiance to the Kim family.

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Photos:North Korea's verbal volleys

April 2012: North Korea's state-run news agency reported that "the moment of explosion is approaching fast" and promised "merciless" strikes against the United States. "The U.S. had better ponder over the prevailing grave situation," it said. Later that month, Pyongyang launched a long-range rocket that broke apart and fell into the sea. The launch came during preparations for a grand party that celebrated the 100th anniversary of the birth of Kim Il Sung, the founder of North Korea.

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Photos:North Korea's verbal volleys

November 2011: North Korea's military threatened to turn the capital of South Korea into a "sea of fire," according to South Korea's Yonhap News Agency.

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Photos:North Korea's verbal volleys

2009: After the U.S. pledge to give nuclear defense to South Korea, Pyongyang threatened a "fire shower of nuclear retaliation."

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Photos:North Korea's verbal volleys

2002: U.S. President George W. Bush includes North Korea in an "axis of evil" with Iran and Iraq, which North Korea brushes off as a "little short of a declaration of war." North Korea reportedly threatened to "wipe out the aggressors." That year, North Korea also threatened to kick out international inspectors who were in the country to monitor its compliance with global nuclear nonproliferation agreements.

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On Thursday, the South Korean military said North Korea fired two short-range ballistic missiles. They were fired from North Hwanghae province, south of Pyongyang, toward the sea east of the Korean Peninsula, the South Korean Joint Chiefs of Staff said. The South tracked the projectiles and is monitoring the situation, it said.

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Also Thursday, the agency published a statement that "all agreements on economic cooperation and business exchanges adopted by North and South are invalid."

This comes after Seoul last month ordered the closure of the Kaesong industrial complex, a rare symbol of cooperation between the two Koreas.

Describing the shuttering of Kaesong as a "unilateral" move, KCNA said Pyongyang "will completely liquidate all South Korean companies and relevant assets" within its borders.

Seoul condemned the suspension of economic ties Thursday, with the Unification Ministry saying in a statement it would "never accept" the move, which it described as a "provocative action."

The statement added it would hold Pyongyang responsible for any damage to South Korean assets north of the border.

Military drills

Meanwhile, around 17,000 U.S. military personnel and 300,000 South Korean troops are taking part in what the South Korean Defense Ministry described as the "largest ever" joint military exercises.

North Korea on Sunday warned it would make a "pre-emptive and offensive nuclear strike" in response to the joint exercises. On Friday, KCNA reported that North Korean leader Kim Jong Un had ordered more tests to improve his country's nuclear attack capability.

The South Korean military spokesman said the two allies were "closely monitoring" signs of North Korean provocation.

"As of now, there are no direct signs of provocation, but we are planning to continuously strengthen surveillance," Defense Ministry spokesman Moon Sang-gyun said.

Last week, KCNA reported that the North Korean leader said his country's "nuclear warheads need to be ready for use at any time."

"Under the extreme situation that the U.S. imperialist is misusing its military influence and is pressuring other countries and people to start war and catastrophe, the only way for our people to protect sovereignty and rights to live is to strengthen the quality and quantity of nuclear power and realize the balance of power," Kim said, according to the KCNA.

While Pyongyang often issues saber-rattling statements during annual U.S. and South Korean joint exercises, "this year the level of anger is much greater," says Mike Chinoy, a former CNN senior international correspondent and the author of "Meltdown: The inside story of the North Korean nuclear crisis."

Chinoy told CNN he was concerned if North Korea "takes even a modest military step, the South will feel obliged to respond."